Field of Invention
For substantially fifty years Sikorsky model helicopters have been identified as single main rotor type aircraft wherein three of more rotor blades are separately attached to arms extending from a central hub, and wherein the blade connections allowed full articulated flight motion to each blade. That is, each blade support includes a set of metal bearings postioned about an axis extending radially from the hub. The axis is called the feathering axis, and blades so supported are able to be rotated by the pilot operated control linkage in order to change the pitch or angle of attack of the blade. A second set of metal bearings are located coincident with the first set, at the blade root, inboard of the feathering bearings and in substantially a horizontal plane. These bearings act as a hinge joint, and allow the blade to make vertical excursions, either flapping upwards in flight or drooping downwards when stationary. A third set of metal bearings are located coincident with, and perpendicular to, the second set. These bearings act as a vertical hinge to allow blade motion in a horizontal (or inplane) direction, either "leading" when in the direction of rotation, or "lagging" when in the opposite direction. For helicopters destined to be parked in confined spaces, or transported to holding areas in small ship elevators, an additional and substantially vertically oriented fold hinge joint is added to the blade at a radial station outboard of the aforementioned three sets of bearings. Manual or automatic systems are used to fold blades into a predetermined nested arrangement. U.S. Pat. No. 3,097,701 to Buivid illustrates a typical Sikorsky rotor head incorporating all of the above features. Prior to pilot initiation of blade folding it is necessary that the rotor be stopped and that an indexing system be actuated to place the rotor head in its predetermined orientation. Each fold hinge pin has been geometrically designed to allow its associated blade path to be free from obstruction by other blades or parts of the aircraft. Rotor stopping also allows centrifugally operated droop and anti-flap stops to engage the blade and restrain it from vertical motion, thus preventing the articulated blade from striking the fuselage when centrifugal loading has decayed, or from rising under the influence of a wind gust. These mechanisms thus work as a pair to restrain the blade from motion about only one of its three axes. Secondly, prior to initiation of blade folding, it is necessary to place each blade and fold hinge joint in a predetermined angular position relative to the longitudinal axis of the aircraft. This is also for the purpose of assuring that the blade follows its programed path to its specific fully folded position. A rotor positioning system indexes the rotor hub arms to a pre-oriented in-plane position. The lag dampers, by a system of differential areas, position the blade and fold hinge in respect to the hub arms to a pre-oriented in-plane position. It is necessary to address and restrain the blade from its third axis of freedom, namely the pitch axis, in order to avoid severe loads or damage being placed on the control linkages to the blade, and prevent blade from striking the ground during folding. The mechanism widely used, and commonly called the "pitch lock" is illustrated in Buivid U.S. Pat. No. 3,097,701 and constitutes a retractable pin used to engage a rotatable blade portion to a non-rotatable portion. The pitch lock does not restrain the blade from flapping or lagging motions. It should be noted that these three separate and independent mechanisms are used to respectively restrain each blade from motion about its respective bearing axis, and only that axis. A further illustration of the prior art pitch lock on a Sikorsky model helicopter is found in U.S. Pat. No. 2,405,777 to Buivid. Other manufacturers' helicopters have used similar bearing and fold hinge configurations with external pitch locks connecting movable pitch shafts to stationary housings. U.S. Pat. Nos. 3,153,455 to Mosinskis, 3,187,818 to Barrett et al and 3,247,907 to Mosinskis, all illustrate such a pitch lock.
A variation to the external pitch lock for the Sikorsky type helicopter comprising an internal spline engagement between a movable blade sleeve and stationary spindle is shown in U.S. Pat. Nos. 3,369,611 and 3,438,446, both co-inventions of applicant Ferris. As an alternate to the external pitch lock pin configuration, the spline configuration may be favored because of its internal location, decreasing the vulnerability to external damage, decreased aerodynamic drag and reduction of the number of crowded components in the confined rotor area. To receive these advantages, the designer must acknowledge that greater loads will be imposed on the spline configuration than on the external pin due to the closer proximity of the spline to the pitch axis and its therefore shorter moment arm.
A further alternate pitch lock system is illustrated relative to a Sikorsky type helicopter in U.S. Pat. Nos. 3,764,229 to Ferris et al and 3,765,794 to Kudasch et al. While the structure depicted in these patents was incorporated into a Sikorsky rotor head including an elastomeric bearing blade support, as depicted in U.S. Pat. No. 3,853,426 to Rybicki, the configuration can also be utilized in rotor heads having separate bearings for feathering, flapping, and in-plane blade motions. A disadvantage of this alternate configuration is the compromising of the capability to predetermine the precise folded position of each blade, as well as its path to such position due to the static deflection of the control. This is also due to the large number of related parts between the blade fold hinge and the lock mechanism, each with its range of manufacturing tolerance. Deviations between possible blade positions can vary up to several degrees, and when in their folded positions may not be allowable, depending on the restricted size envelope for folded blades.
With the advent of the elastomeric type rotor head, as depicted in U.S. Pat. No. 3,782,854 to Rybicki, coupled to the requirement for precise blade folded positioning, a pitch lock system other than that illustrated in U.S. Pat. Nos. 3,764,229 and 3,765,794 was required. A further complication for the system was a compatability requirement with the standard Sikorsky system for blade interchangeability as described in U.S. Pat. No. 3,008,525 to Jensen. Under Jensen's system, blades deviating due to manufacturing tolerances are brought into track by selective adjustment to the length of the associated pitch change rod. Since this system would otherwise result in variations of several degrees for a blade, it was a further requirement that the pitch lock incorporate an adjustment feature that could interrelate with the tracking adjustment, and any additional adjustment to collective setting of the rotor to improve flight condition. A still further requirement for our fold restraint system was the capability of manual pitch lock engagement in the event of failure of the automatic activation system.
The prior art Sikorsky internal spline system described in U.S. Pat. Nos. 3,369,611 and 3,438,446 was considered and rejected because of its short moment arm, and the magnitude of the high folding loads, highwind loads, and rolling of the aircraft on deck of a ship at sea. Manual engagement of this internal spline pitch lock is extremely difficult because of its internal inaccessability. Further, we have found the addition of an adjustment feature to this internal spline system to be extremely complex. An illustration of the spline lock system in the elastomeric bearing rotor application is found in U.S. Pat. No. 4,028,001 to Watson. One desirable apparent feature of such configuration is that the spline pitch lock functions not only to lock the blade at a preselected pitch setting, but may also restrain the blade from excursions in flapping and in-plane directions. Such restraint is necessary not only to maintain a predetermined path during folding and to hold the blade in its folded position, but might also be needed to prevent excessive bearing shear motions, which could rupture the elastomeric bearing. This concern was an added consideration for the designer, introduced by the use of the elastomeric type bearing, which achieves the full articulation provided in the past by three separate bearing sets.
With the above objectives and requirements in mind the configuration described hereinafter was conceived as a pitch, flap and lead-lag restraint system for the elastomeric rotor system of the type depicted in U.S. Pat. No. 3,782,854 to Rybicki. Key elements have been selected from the prior art pitch lock systems, then selectively arranged and oriented to co-act for the first known time with known anti-flap and droop stops, and provided with the addition of a tolerance accommodating and adjustment system. Further, manual provisions have been provided for engaging the pitch lock in case of mechanical failure of the system. Thus, an alternate to the specific fold restraint system for U.S. Pat. No. 4,028,001 to Watson is provided which utilizes all the advantages described therewith, and in addition achieves the stated new benefits.
Other objects and advantages of the present invention may be seen by referring to the following description and claims, read in conjunction with the accompanying drawings.